It is known that underwater acoustic transducers are used extensively both in military and civil applications. The majority of these transducers are based on the use of piezoelectric and electrostrictive materials as the active material in the transducer with functions of acoustic signal detectors, resonators, acoustic projectors and ultrasonic imaging. Typical civil applications include oceanographic survey, geographical exploration, depth sounding and fish finding, whereas in military they are used in active sonar, obstacle avoidance, mine hunting, underwater communication etc.
In the case of sonar applications, the transducer is a reciprocal device, such that when electricity is applied to the transducer, a pressure wave is generated in water, and when a pressure wave impinges on the transducer, electricity is developed. The transducer may be employed as a transmitting device (projector), a listening device (hydrophone), or both. Depending on various applications, several designs of projectors and hydrophones are available in patents and commercially.
Most of the transducers are omni directional in performance and directionality is by and large obtained by two methods. One is by the modification of the driver as explained, for example, in U.S. Pat. Nos. 4,754,441 and 6,614,143. The first patent describes the use of multiple curved shells driven by a ring or corresponding number of attached piezoelectric or magnetostricitive type rod or bar drivers which together take on the form of regular polygon. The second patent explain the complicated design of an electro active device with first and second electro active substrates each having first and second opposed continuous planar surfaces wherein each of the first opposed surfaces have a polarity and each of the opposed surfaces have an opposite polarity. For many of the common purposes, such complicated design aspects of the electro active driver may be avoided by a simple yet novel technique.
The other less tedious and less complex method is by the use of acoustic reflectors. It is necessary to utilize walls which reflect sound waves in a number of devices. The surface of separation between two materials having different acoustic impendence is known to form a good acoustic reflector. Water has relatively high acoustic impedance, while many light materials such as gas, cork, or cellular material have impedance much lower than water and have therefore been used for submerged reflectors. Some of the material that are used in the present context include celtite and corprene, and some specialized elastomers marketed under trade names familiar in the art. A plurality of apparatus has been employed in the past in a series of similar applications. For example, U.S. Pat. No. 3,756,345 explains the use of precompressed balsa wood as acoustic reflector or decoupler providing excellent insertion loss. A reflector made of a stack of metallic mesh members mounted between two rigid plates made of stratified synthetic resin within an enclosure is detailed in U.S. Pat. No. 3,901,352. This however, needs an intricate design of intermeshing of high modulus filaments.
In U.S. Pat. No. 4,090,171, an underwater acoustic reflector for use at elevated hydrostatic pressures comprising a plurality of thin paper laminate assembled in an integrated stack enclosed by a thin-walled gas confining wrapping, and a waterproof jacketing has been described, however requiring a plurality of materials and assembly methods. Even the use of electrochemistry to produce bubbles by using electrodes and aqueous electrolyte solution, which forms a reflective surface, has been specified in U.S. Pat. No. 4,197,920. Apparently, this necessitates for electrodes and a constant supply of current for its use as a reflector, which may not be suitable for the aforesaid applications. The use of hollow gas-filled sphere in the production of corner reflectors of passive acoustic navigation aid is noted in U.S. Pat. No. 4,126,847, which would require the intricate requisite for gas filled spherical membranes arranged over three substantially mutually perpendicular surfaces, as in a corner reflector. Another patent explains the use of inactive ceramic coating to induce directionality (U.S. Pat. No. 4,754,441). Various acoustic decouplers in the past have been designed to provide, in conjunction with a signal conditioning plate, the proper impedance backing for one or more hydrophones included in the array and to isolate or decouple structure borne noise as explained in patent by Eynck (U.S. Pat. No. 4,982,385). In U.S. Pat. No. 5,099,457, details are given about an acoustic wave reflector capable of working under deep submersion using a sheet of air set up between a reflecting plate and a perforated plate and having a rubber bladder which, under the effect of the pressure of the water, feeds the sheet of air through the perforated plate.
A Flextentional transducer (FT) has been made directional using a plurality of wells in U.S. Pat. No. 5,764,782. While many of the prior art reflectors are exceptionally efficient, many of the acoustic reflecting material used heretofore or complicated and tedious to fabricate or may not retain there desirable reflecting properties at elevated hydrostatic pressure. Most of the widely used reflecting materials are visco-elastic polymers in micro cellular form or that containing hard, air-encapsulated bubble like materials. Hence, it was observed that desirable low pressure acoustic properties of many of them are often severely impaired after they are subjected to high hydrostatic pressure. Moreover, this function becomes even more difficult as one resort to lower frequencies of operation. The Inventors hereof have recognized the need for acoustic reflector that can easily fabricated with readily available material, but that will undergo performance degradation with increasing hydrostatic pressure or water absorption, such a material or device will have great implications in sonar used for both civil and military operations.